1. Field of the Invention
The present invention relates to a water treatment apparatus. More particularly, the present invention relates to a water treatment apparatus suitable for converting seawater to freshwater.
2. Description of the Related Art
Seawater contains 4 to 5 mg/l of boron. Therefore, in order to obtain drinking water from seawater, the concentration of boron needs to be reduced to 1 mg/l or less. Conventionally, a two-step treatment using a composite reverse osmosis membrane has been proposed for the purpose of sufficiently reducing the concentration of boron (JP 10(1998)-305216 A, JP 8(1996)-206460 A, etc.). However, according to this treatment method, although the concentration of boron is reduced, other necessary ions are also removed. Accordingly, ions need to be added after treatment. Typically, a spiral module formed by winding a composite reverse osmosis membrane around a water-collecting pipe is used for water treatment. Conventional water treatment methods require a large number of such modules, which is economically problematic.
Therefore, with the foregoing in mind, one aspect of the present invention is to provide a water treatment apparatus for economically treating water that can sufficiently reduce boron while maintaining ions necessary for living bodies.
In order to achieve this, the water treatment apparatus according to the present invention includes a plurality of composite reverse osmosis membrane modules arranged in multi-stages, each of the modules including a porous support and a polyamide skin layer formed on the porous support, the plurality of modules including a final-stage module and at least one module preceding the final-stage module (hereinafter, referred to as xe2x80x9cpre-final modulexe2x80x9d). The apparatus is characterized in that some permeated water obtained from the at least one pre-final module is supplied to the final-stage module, and the remaining permeated water is discharged from or recovered in the apparatus along with permeated water obtained from the final-stage module.
According to this apparatus, when raw water to be treated is seawater, a concentration of boron can be reduced while maintaining the levels of ions necessary for living bodies. In addition, the apparatus requires fewer modules.
In the apparatus according to the present invention, it is preferable that the permeated water from the final-stage module and the permeated water that is not supplied from the at least one pre-final module to the final-stage module are mixed with each other to be discharged or recovered.
In this apparatus, seawater (a TDS concentration of 4.1%, a boron concentration of 5 mg/l, a temperature of 28xc2x0 C.) was supplied to the first pressure vessel from its one end at a pressure of 6.3 MPa. A recovery ratio (the amount of the permeated water/the amount of the feed water) in the first pressure vessel was 50%. In the permeated water on the raw water side (upstream) of the first pressure vessel, the concentration of boron was 0.55 mg/l and the concentration of TDS was 110 mg/l. On the other hand, in the permeated water on the concentrate side (downstream) of the first pressure vessel, the concentration of boron was 10.9 mg/l and the concentration of TDS was about 490 mg/l. Further, a volume ratio (X:Y) of the amount of the permeated water on the upstream and the amount of the permeated water on the downstream was 1:1.4. Then, the pH of the permeated water on the downstream was adjusted to 9.5 using sodium hydroxide and supplied to the second pressure vessel at a pressure of 0.8 MPa. A recovery ratio in the second pressure vessel was 85%. In the permeated water obtained from the second pressure vessel, the concentration of boron was 0.7 mg/l and the concentration of TDS was 16 mg/l. Then, the permeated water (C) on the upstream side of the first pressure vessel and the permeated water (D) obtained from the second pressure vessel were mixed with each other (mixing ratio by volume C:D=1.6:1). In the mixed water thus obtained, the concentration boron was 0.6 mg/l and the concentration of TDS was 74 mg/l. The quality of the mixed water was satisfactory for drinking water and in addition, there was no need to add ions separately.
In the apparatus according to the present invention, the permeated water supplied to the final-stage module is preferably alkaline, for example, having a pH of 8 to 12, preferably 9 to 12, and more preferably 9 to 11. The reason for this is that boron in the permeated water thus adjusted in alkaline pH is in a dissociated state and can be more easily removed.
In the apparatus according to the present invention, the permeated water supplied to the final-stage module is preferably discharged from a concentrate side of at least one module supplying the permeated water to the final-stage module. This can help an ion concentration of the permeated water discharged from or recovered in the apparatus as a whole to be more efficiently reduced.
In one example, the above-mentioned water treatment apparatus has the following structure so that the permeated water discharged from the concentrate side is supplied to the final-stage module. That is, the water treatment apparatus further includes a pressure vessel. The apparatus is characterized in that a plurality of pre-final modules are provided as the above-mentioned at least one pre-final module, each of the plurality of pre-final modules is a spiral module formed by winding a composite reverse osmosis membrane around a water-collecting pipe, the plurality of pre-final modules are connected with each other by connecting their water-collecting pipes, the plurality of pre-final modules thus connected are contained in the plurality of pressure vessels, raw water to be treated is supplied to and permeated water is discharged from one end of the pressure vessel, concentrated water and permeated water are discharged from the other end of the pressure vessel, and the permeated water discharged from the other end is supplied to the final-stage module. In the apparatus according to this example, the method for taking out the permeated water from both the end of the pressure vessel is not specifically limited. For example, an amount of the permeated water may be adjusted by bulbs provided in outlet pipes at both the ends of the pressure vessel. Further, an interior space of the connected water-collecting pipes may be divide into two separate spaces by a partition to separate permeated water on a raw water side and permeated water on a concentrate side. In this case, the volume ratio (X:Y) of the amount of the permeated water on the raw water side (upstream) and the amount of the permeated water on the concentrate side (downstream) is in the range of, for example, 1:5 to 9:1, preferably 1:1.5 to 9:1, and more preferably 1:1 to 4:1.
In another example, the water treatment apparatus has the following structure so that the permeated water discharged from the concentrate side is supplied to the final-stage module. That is, the water treatment apparatus further includes a plurality of pressure vessels arranged in multi-stages, the plurality of pressure vessels including a first-stage pressure vessel and at least one pressure vessel subsequent to the first-stage pressure vessel. The apparatus is characterized in that a plurality of pre-final modules are provided as the above-mentioned at least one pre-final module, each of the plurality of pre-final modules is a spiral module formed by winding a composite reverse osmosis membrane around a water-collecting pipe, the plurality of pre-final modules are connected with each other by connecting their water-collecting pipes, the plurality of pre-final modules thus connected are contained in the pressure vessel, the first-stage pressure vessel is supplied with raw water to be treated, the at least one pressure vessel subsequent to the first-stage pressure vessel is supplied with concentrated water discharged from at least one preceding pressure vessel, and the final-stage module is supplied with permeated water discharged from the at least one pressure vessel subsequent to the first-stage pressure vessel.
More specific example of the apparatus according to this example includes three pressure vessels arranged in three stages, and permeated water from a second-stage pressure vessel and/or a third-stage pressure vessel is supplied to a final-stage composite reverse osmosis membrane.
In the water treatment apparatus according to the present invention, it is preferable that the at least one pre-final module has a salt rejection of at least 99% and a permeate flux of at least 0.2 m3/m2xc2x7day when the apparatus is operated using as a feed solution a 3.5 wt % salt water at a pH of 6.5, a water temperature of 25xc2x0 C., and an operational pressure of 5.5 MPa. It is more preferable that these modules have a salt rejection of at least 99.5% and a permeate flux of at least 0.3 m3/m2xc2x7day under the same conditions.
In the water treatment apparatus according to the present invention, it is preferable that the at least one pre-final module has a boron rejection of at least 80% when the apparatus is operated using as a feed solution a 3.5 wt % salt water containing 5 ppm of boron at a pH of 6.5, a water temperature of 25xc2x0 C., and an operational pressure of 5.5 MPa. It is more preferable that these modules have a boron rejection of at least 90% under the same conditions.
In the water treatment apparatus according to the present invention, it is preferable that the final-stage composite reverse osmosis membrane module has a salt rejection of at least 98% and a permeate flux of at least 0.5 m3/m2xc2x7day when the apparatus is operated using as a feed solution a 0.05 wt % salt water a pH of 6.5, at a water temperature of 25xc2x0 C., and an operational pressure of 0.75 MPa. It is more preferable that the module has a salt rejection of not less than 99.0% and a permeate flux of not less than 0.7 m3/m2xc2x7day under the same conditions.
It should be noted here that the term xe2x80x9cpermeate fluxxe2x80x9d as used herein refers to an amount of a solution that permeates a unit area of the composite reverse osmosis membrane per unit time. Further, a rejection for a substance to be rejected (a rejection for salt, a rejection for boron, etc.) is defined by the following equation:
Rejection(%)=(1xe2x88x92(the concentration of the substance in the permeate solution/the average concentration of the substance in the feed solution))xc3x97100. 
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.